1. Field of the Invention
The present invention relates to a range gearbox for motor vehicles and that is intended to be connected to the output side of a basic gearbox. The device includes a planetary gear (arrangement or set), enclosed in a housing, with a sun wheel that is arranged on an input shaft and engages with planet wheels. The planet wheels are carried by a planet wheel carrier connected to an output shaft and engaged with an axially displaceable ring gear by a coupling means or mechanism. The coupling means or mechanism is lockable either in a first coupling position in which it is non-rotatably connected to the housing in order to establish gearing between the input shaft and the output shaft, or in a second coupling position in which it is connected non-rotatably to the input shaft or the output shaft in order to establish direct drive between the shafts.
2. Background
It is typical for vehicle transmissions to include a basic gearbox and a range gearbox coupled to the output side thereof. This is especially true in heavy-duty vehicles in which it is necessary to provide a large number of gearings in order to make it possible to drive within a speed range favorable to the engine when the vehicle is driven at varying speed and under varying load. A range gearbox is usually of a two-shift type; this means that the number of possible gear ratios is doubled when a range gearbox is coupled together with a main gearbox. In one of the two gear ratios occurring in a range gear arrangement, the input shaft is connected to the output shaft, and no gearing takes place in the range gearbox. This position is usually referred to as the high range. In the other gear ratio, the input shaft is connected to the output shaft via a sun wheel, planet wheel and ring gear arrangement, and gearing between the input and output shafts takes place. This position is usually referred to as the low range.
A range gearbox of the type described above is disclosed, for example, in GB 2151316. The device disclosed therein includes a sun wheel connected rigidly to the input shaft, and a planet wheel carrier connected rigidly to the output shaft. The planetary gearbox also has an axially displaceable ring gear which, by coupling means, is lockable either against rotation relative to said housing in order to establish gearing between the input shaft and the output shaft, or against rotation relative to the shafts in order to establish direct drive between the input shaft and the output shaft.
It is desirable that the shifting movement between low range and high range be able to be performed as rapidly and smoothly as possible in order that the speed of the vehicle not be reduced unnecessarily during the shifting operation. As an example, this is of particular importance if the shifting is taking place on an upward gradient. Although shifting is pneumatically assisted, the mass moment of inertia to be accelerated during synchronizing is relatively great. Moreover, the ring gear and associated coupling means are to be moved during the shifting operation, from one coupling position to the other.